This invention refers to a reluctance synchronous electric machine fed by an electronic power circuit by vectorial control of the feed current.
According to the state of art preceding this invention, the system formed by an alternate current electric machine (and therefore, in particular, by a reluctance synchronous machine), by the pertinent electronic feed circuit (inverter) and by its circuit for a vectorial control (in amplitude and phase) of the feed current, cannot be designed such as to exploit in the more favorable manner both the characteristics of the machine and the characteristics of the inverter.
In effect it is noticed that, in order to suitably exploit the characteristics of the inverter, the machine should be proportioned for delivering a maximum power greater than that really allowed by the inverter, whereas on the contrary, in order to suitably exploit the characteristics of the machine, the inverter should be proportioned for delivering a maximum power greater than that allowed by the machine. This is particularly burdensome in the frequent case in which the machine is required to deliver a constant torque (corresponding to a prefixed maximum value) at any speed lower than an intermediate speed at which, with said prefixed constant torque, a fixed maximum value of power is delivered, whereas the machine should deliver a constant power, corresponding to the above cited fixed maximum value of power, within a large range of speed over said intermediate speed. In order that this may be possible, the inverter should be proportioned for feeding the nominal current needed for producing the prefixed maximum torque, as well as for feeding the maximum voltage needed for delivering the prefixed maximum power up to the maximum speed. The product of the nominal current multiplied by the maximum voltage is a socalled "dimensioning power", for which the inverter should be proportioned, and it may be much greater than the maximum mechanical power intended to be delivered.
This may be well understood with reference to the vectorial diagram of FIG. 1. It should be noted that the distribution of sinusoidal magnetomotive force generated by the stator winding of the machine may be construed as resolved in two sinusoidal distributions of magnetomotive force, directed along the direct axis (d) of the rotor and the quadrature axis (q) of the rotor, respectively. On their turn, these distributions of magnetomotive force may be construed as produced by two conductor distributions through which flow electrical currents i.sub.d and i.sub.q, respectively. The magnetic fluxes .lambda..sub.d and .lambda..sub.q, directed along the axes d and q, respectively, which chain the two windings corresponding to said conductor distributions, are given by the respective currents i.sub.d and i.sub.q multiplied by the respective self-inductances L.sub.d and L.sub.q, which are characteristic of the magnetic construction of the rotor: .lambda..sub.d =L.sub.d .multidot.i.sub.d ; .lambda..sub.q =L.sub.q .multidot.i.sub.q. The total vectors of current (i) and magnetic flux (.lambda.) result from the vectorial sum of the respective components (i.sub. d, i.sub.q and .lambda..sub.d, .lambda..sub.q) along the axes d and q, respectively, of the rotor. The angular speed of the rotor with respect to the stator will be indicated as .omega.. The analysis of the system shows that, if the resistive drops are disregarded, and a stationary distribution of the magnetic state is taken into account, the resulting voltage vector v, which is suitable for giving rise to the current vector i into the stator winding, is in advance and in quadrature with respect to the flux vector .lambda.. The vector v subtends with the vector i an angle .phi.. By denoting as T the produced mechanical torque, and henceforth .omega..multidot.T the delivered mechanical power, the apparent input electrical power is v.multidot.i, the active input electrical power is v.multidot.i.multidot.cos.phi.=.omega..multidot.T, and the amplitude of the voltage v needed for producing the magnetic fluxes involved is v=.omega..sqroot..lambda..sub.d.sup.2 +.lambda..sub.q.sup.2. The general formula which gives the torque is T=.lambda..sub.d .multidot.i.sub.q -.lambda..sub.q .multidot.i.sub.d.
It may be shown that the maximum torque which may be produced without overcoming a maximum voltage V.sub.M is obtained when .vertline..lambda..sub.d .vertline.=.vertline..lambda..sub.q .vertline.=V.sub.M /.omega..sqroot.2, and it is T.sub.max =(1/L.sub.q -1/L.sub.d).multidot.V.sub.M.sup.2 /2.omega..sup.2, whereby the maximum power is P.sub.max =(1/L.sub.q -1/L.sub.d).multidot.V.sub.M.sup.2 /2.omega..
As a consequence, the maximum voltage V.sub.M which should be applied in order to obtain the power P.sub.max increases with the maximum angular speed .omega. at which said power is to be delivered, namely with the extent of the range wherein the operation at constant power is required. If I.sub.o is the nominal current foreseen for the machine, the proportioning power (for which the inverter should be proportioned) is V.sub.M .multidot.I.sub.o : it is much greater than the mechanical power .omega..multidot.T which may be delivered by the system, and it increases with the extent of the speed range wherein an operation at constant mechanical power is required.